Hitherto, a planar light circuit (PLC) formed on a substrate is used in the optical communication field. The planar light circuit is used mainly as a silica-based material of an optical waveguide. For example, a planar light circuit of an array waveguide grating (AWG), a splitter, or the like is used as a backbone part in optical communication.
Recently, in an attempt to realize a smaller system by mounting an active optical device and a passive optical device on a common PLC substrate is being made. For example, a novel optical device such as a wavelength-variable light source in which a compound semiconductor amplifier (SOA) is hybrid-packaged on a silica-based PLC is being developed.
However, as a demanded function is becoming more complicated and sophisticated, the device dimension and consumption power of the planar optical circuit are increasing. Consequently, it is becoming difficult to improve the function or performance of an optical circuit using the conventional silica-based PLC.
Accordingly, a technique of forming an optical waveguide on an SOI (Silicon On Insulator) substrate using the silicon microfabrication technique such as a silicon wire waveguide or a photonic crystal (PC) is being studied. Development of a small backbone part characterized by low power consumption is being examined using the silicon microfabrication technique.
A silicon wire waveguide has a core layer formed by silicon and a cladding layer made of SiO2 or its dielectric. Since the relative index difference between silicon and SiO2 or its dielectric is larger than that of PLC, the width of an optical waveguide or bending radius of an optical waveguide can be made smaller than that of a PLC. Therefore, the dimensions of a silicon wire waveguide using the silicon microfabrication technique can be made smaller than those of a conventional PLC. By using such a silicon wire waveguide, a small-sized optical circuit can be realized. The reason will be described below.
In a conventional silica-based PLC, the relative index difference between a core layer and a cladding layer is at most about 5%, and the limit of the bending radius is about 500 μm. On the other hand, in the silicon wire waveguide, the relative index difference is 40% or higher, and the bending radius can be reduced to a few microns. However, when the relative index difference is increased, the size of the core layer has to be reduced in order to satisfy a single mode parameter of propagation light.
It causes the difference in the spot size between an optical fiber or another waveguide device and a silicon wire waveguide, so that loss of light occurs.
To solve the problem, a method of reducing the spot size of light which is incident on the silicon wire waveguide has been proposed. For example, it is proposed to reduce the width of a waveguide which guides incident light to the silicon wire waveguide in a tapered shape.
Japanese Laid-open Patent Publication No. 2004-151700